This invention relates to the diffusion bonding of metals such as titanium, and its alloys, and which may be subsequently formed into structures by superplastic forming, and is particularly concerned with the production of structural members having preselected shapes such as T-caps by diffusion bonding and superplastic forming.
A number of alloys exhibit superplasticity and are capable of being subjected to superplastic forming to produce parts of predetermined shapes. Superplasticity is the capability of a material to develop unusually high tensile elongation with reduced tendency toward local necking during deformation. Prior to such superplastic forming, diffusion bonding of the metal workpieces is carried out to bond the workpieces in certain preselected areas, to permit superplastic forming to be carried out in the unbonded areas of the workpieces.
Structures have been successfully produced from a number of titanium-based alloys by the Superplastic Forming/Diffusion Bonding (SPF/DB) process. Such structures are producible because such metals or alloys exhibit the two essential physical properties required for SPF/DB, namely ability to be diffusion bonded and superplasticity.
Diffusion bonding refers to the solid-state, metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to effect intimate surface contact and cause comingling of atoms at the joint interface.
Examples of metals which can be diffusion bonded and which have superplasticity characteristics include titanium, zirconium, refractory metals, and alloys thereof. Aluminum may also be suitable for this purpose, since recent developments indicate that aluminum and its alloys can be diffusion bonded, as well as being capable of superplastic forming.
U.S. Pat. No. 3,927,817 discloses a method for fabrication or structures in which metal blanks, preferably of a titanium alloy, are joined at selected areas by diffusion bonding at elevated temperatures and pressures, and then subjected to superplastic forming to form a desired structure. The metal blanks are first treated at selected areas with a stopoff material, such as yttria, boron nitride, graphite, or alumina, to prevent bonding at such treated areas during diffusion bonding. During superplastic forming the metal blanks are expanded at the treated (unbonded) areas into contact with shaping members by increasing the internal pressure, preferably with an inert gas, thus forming an expanded structure of a desired shape, essentially in a single operation.
Thus, after the bonds between adjacent metal blanks are formed during diffusion bonding, inert gas pressure, such as argon or helium, is applied to the interior network to superplastically form the unbonded protions of the adjacent metal sheets.
U.S. Pat. Nos. 4,303,570 and 4,331,284 are further illustrative of the production of diffusion bonded and superplastically formed structures and techniques employed therein.
It is an object of the present invention to provide a process for producing structural members incorporating preselected shapes.
Another object of the invention is the provision of procedure for the production of structural members incorporating preselected shapes, particularly T-caps, by diffusion bonding and superplastic forming.
A still further object is the provision in the above-noted procedure, of means in the form of special tooling apparatus to facilitate production of structural members incorporating preselected shapes such as symmetrical T-caps, by the SPF/DB process.